1. Field of the Invention
The present invention relates to an optical pickup and an optical disk apparatus including an optical pickup that are capable of recording and replaying information signals using one objective lens for three different types of disk recording medium.
2. Description of the Related Art
As next-generation optical disk format, a known optical disk capable of high-density recording (hereinafter referred to as a high-density recording optical disk) in which recording and replay of a signal is carried out using an optical beam having a wavelength of approximately 405 nm generated by a blue-violet semiconductor laser has been proposed. As such a high-density recording optical disk, a disk having a thin protective substrate layer for protecting the signal recording layer of, for example, 0.1 mm has been proposed.
To provide an optical pickup supporting these high-density recording optical disks, it is desirable a provide an optical pickup that has compatibility with optical disks having different formats, such as a known compact disk (CD) having a usable wavelength of approximately 785 nm and a protective substrate layer with a thickness of approximately 1.2 mm or a digital versatile disc (DVD) having a usable wavelength of approximately 660 nm and a protective substrate layer with a thickness of approximately 0.6 mm. In this way, an optical pickup and an optical disk apparatus having compatibility with optical disks having different formats with different disk structures and different laser specifications are in need.
As a known method of recording and replaying an information signal for optical disks having three different types of format, a method of switching among objective lenses included in an optical pickup having a plurality of optical systems corresponding to the usable wavelengths has been proposed.
However, to provide various different optical systems requires a mechanism for switching among various different objective lenses and requires various different components including actuators. Therefore, the overall size of the apparatus is great, and the structure of the apparatus becomes complex.
To simplify the structure, an optical pickup may employ a common optical path and common optical components for all wavelengths. To provide such an optical pickup, the optical path must be the same for all wavelengths and the common objective lens must not generate aberration by optical beams having different wavelengths and different disk thicknesses have to be provided.
With such an optical pickup, when a working distance of at least 0.3 mm is provided for each optical disk, such as a CD having a great usable wavelength and a thick protective substrate layer, the aperture for a high-density recording optical disk having a small usable wavelength and a thin protective substrate layer increases to approximately 4 mm, and the objective lens becomes large and heavy. Therefore, the tracking ability is reduced, and the size of other optical components increases together with the increase in the aperture. Thus, there is a problem in that the overall size of the apparatus increases.
To avoid this problem, an optical beam of a CD or a DVD may be diverged with an optical element and be incident on an objective lens. However, due a reduction in the light intensity caused by the diffraction efficiency and transmittance of the optical element, it has been extremely difficult to carry out excellent recording and replay for an optical disk using three different wavelengths.
For a known optical pickup, for example, refer to Japanese Unexamined Patent Application Publication No. 2003-177226.